With each repetition, my body builds itself.

Nobody's body was built by the first repetition or the last one. It gets built by the fifth, the one in the middle of the middle set, where nothing is dramatic and you're largely thinking about dinner.

Bricklayers know this shape of work. No single brick is the wall. There is no brick at which the wall becomes a wall.

The building is literal, too. Load applied to muscle and bone reads as an instruction to lay down more material, and the body obliges over the following days. Which means the unremarkable rep, done adequately, on an afternoon you didn't want to, is also the instruction. It counts whether or not it felt like anything.